The present invention relates generally to radio frequency (RF) communication systems and more specifically to automatically adaptive digital RF squelch systems.
Squelch systems in the prior art have relied on either a fixed squelch or a manually adjustable squelch. With either type of squelch, two types of errors may occur. If the squelch is set too high, valid signals that contain information otherwise capable of being received by the receiver may not be detected. If the squelch level is set too low, ambient RF background noise will be received by the receiver and mistaken for valid signals. For manually-adjustable systems, an operator must make adjustments to the squelch to optimize the squelch level for a particular RF noise environment. These errors are particularly harmful for equipment used in motor vehicles, where ignition and other noise sources make appropriate squelch levels difficult to predict.
Automatic threshold squelch techniques are known wherein a stored detected noise signal is compared to a current detected noise signal to control a squelch gate. An analog noise detector in the form of an analog integrator is disclosed for example in U.S. Pat. No. 4,411,021 issued to J. H. Yoakum of Motorola. Analog noise detection of the type shown is inherently inflexible as it utilizes high pass filters to isolate noise and the squelch level is set according to signals and noise at the audio signal level.
Squelch systems are also known wherein a set noise reference level is compared to a current intermediate frequency (IF) signal to produce a squelch signal. A squelch system of this type is disclosed for example in U.S. Pat. No. 4,718,115 issued to T. Inoue of NEC Corporation. Squelch signal control of the type shown is also inflexible as it does not adjust the receiver squelch level according to changes in the ambient RF background noise and it is responsive only to noise within the signal frequency range.
Thus, there is a need for an adaptive squelch that sets the receiver squelch level automatically according to changes in the ambient RF background noise environment over a broad frequency range.